Evidence for the involvement of a specific cell wall layer in regulation of deep supercooling of xylem parenchyma

Oil and mucilage idioblasts co-occur in the vegetative organs of Ocotea pulchella (Lauraceae): comparative development, ultrastructure and secretions

This study compares oil and mucilage idioblasts occurring together in the vegetative organs of Ocotea pulchella, a Lauraceae species. Our focus is specifically on the ontogeny and developmental cytology of these secretory cells. Both types of idioblasts originate from solitary cells located in the fundamental meristem, underlying the protodermis. The growth of both types of idioblasts is asynchronous, with the oil idioblasts developing first, but their initiation is restricted to the early stages of organ development. Mucilaginous idioblasts occur exclusively in the palisade parenchyma, while oil idioblasts are scattered throughout the mesophyll, midrib, and petiole of the leaves. The lamellar secretion of mucilage idioblasts is mostly made up of polysaccharides, while the secretion of oil idioblasts is made up of terpenes and lipids. Cupule occurred only in the oil idioblasts, while suberized layers occurred in both types of cells. We found that immature oil idioblasts that are close to each other fuse; mature mucilage idioblasts have labyrinthine walls arranged in a reticulate pattern; the cells close to the oil idioblasts have a pectin protective layer; and the oil idioblasts have a sheath of phenolic cells. In contrast to previous reports, the two types of secretory idioblasts were recognized during the early stages of their development. The results emphasize the importance of combining optical and electron microscopy methods to observe the ontogenetic, histochemical and ultrastructural changes that occur during the development of the secretory idioblasts. This can help us understand how secreting cells store their secretions and how their walls become specialized.

Protective Role of Ice Barriers: How Reproductive Organs of Early Flowering and Mountain Plants Escape Frost Injuries

In the temperate zone of Europe, plants flowering in early spring or at high elevation risk that their reproductive organs are harmed by episodic frosts. Focusing on flowers of two mountain and three early-flowering colline to montane distributed species, vulnerability to ice formation and ice management strategies using infrared video thermography were investigated. Three species had ice susceptible flowers and structural ice barriers, between the vegetative and reproductive organs, that prevent ice entrance from the frozen stems. Structural ice barriers as found in Anemona nemorosa and Muscari sp. have not yet been described for herbaceous species that of Jasminum nudiflorum corroborates findings for woody species. Flowers of Galanthus nivalis and Scilla forbesii were ice tolerant. For all herbs, it became clear that the soil acts as a thermal insulator for frost susceptible below ground organs and as a thermal barrier against the spread of ice between individual flowers and leaves. Both ice barrier types presumably promote that the reproductive organs can remain supercooled, and can at least for a certain time-period escape from effects of ice formation. Both effects of ice barriers appear significant in the habitat of the tested species, where episodic freezing events potentially curtail the reproductive success.

Mechanism of Overwintering in Trees

Boreal trees possess very high freezing resistance, which is induced by short-day length and low temperatures, in order to survive severe subzero temperatures in winter. During autumn, cooperation of photoreceptors and circadian clock system perceiving photoperiod shortening results in growth cessation, dormancy development, and first induction of freezing resistance. The freezing resistance is further enhanced by subsequent low temperature during seasonal cold acclimation with concomitant changes in various morphological and physiological features including accumulation of sugars and late embryogenesis abundant proteins. The mechanism of adaptation to freezing temperatures differs depending on the type of tissue in boreal trees. For example, bark, cambium, and leaf cells tolerate freezing-induced dehydration by extracellular freezing, whereas xylem parenchyma cells avoid intracellular freezing by deep supercooling. In addition, dormant buds in some trees respond by extraorgan freezing. Boreal trees have evolved overwintering mechanisms such as dormancy and high freezing resistance in order to survive freezing temperatures in winter.

Vessel-associated cells in angiosperm xylem: Highly specialized living cells at the symplast-apoplast boundary

BACKGROUND

Vessel-associated cells (VACs) are highly specialized, living parenchyma cells that are in direct contact with water-conducting, dead vessels. The contact may be sparse or in large tight groups of parenchyma that completely surrounds vessels. VACs differ from vessel distant parenchyma in physiology, anatomy, and function and have half-bordered pits at the vessel-parenchyma juncture. The distinct anatomy of VACs is related to the exchange of substances to and from the water-transport system, with the cells long thought to be involved in water transport in woody angiosperms, but where direct experimental evidence is lacking.

SCOPE

This review focuses on our current knowledge of VACs regarding anatomy and function, including hydraulic capacitance, storage of nonstructural carbohydrates, symplastic and apoplastic interactions, defense against pathogens and frost, osmoregulation, and the novel hypothesis of surfactant production. Based on microscopy, we visually represent how VACs vary in dimensions and general appearance between species, with special attention to the protoplast, amorphous layer, and the vessel-parenchyma pit membrane.

CONCLUSIONS

An understanding of the relationship between VACs and vessels is crucial to tackling questions related to how water is transported over long distances in xylem, as well as defense against pathogens. New avenues of research show how parenchyma-vessel contact is related to vessel diameter and a new hypothesis may explain how surfactants arising from VAC can allow water to travel under negative pressure. We also reinforce the message of connectivity between VAC and other cells between xylem and phloem.

Bordered pits in xylem of vesselless angiosperms and their possible misinterpretation as perforation plates

Vesselless wood represents a rare phenomenon within the angiosperms, characterizing Amborellaceae, Trochodendraceae and Winteraceae. Anatomical observations of bordered pits and their pit membranes based on light, scanning and transmission electron microscopy (SEM and TEM) are required to understand functional questions surrounding vesselless angiosperms and the potential occurrence of cryptic vessels. Interconduit pit membranes in 11 vesselless species showed a similar ultrastructure as mesophytic vessel-bearing angiosperms, with a mean thickness of 245 nm (± 53, SD; n = six species). Shrunken, damaged and aspirated pit membranes, which were 52% thinner than pit membranes in fresh samples (n = four species), occurred in all dried-and-rehydrated samples, and in fresh latewood of Tetracentron sinense and Trochodendron aralioides. SEM demonstrated that shrunken pit membranes showed artificially enlarged, > 100 nm wide pores. Moreover, perfusion experiments with stem segments of Drimys winteri showed that 20 and 50 nm colloidal gold particles only passed through 2 cm long dried-and-rehydrated segments, but not through similar sized fresh ones. These results indicate that pit membrane shrinkage is irreversible and associated with a considerable increase in pore size. Moreover, our findings suggest that pit membrane damage, which may occur in planta, could explain earlier records of vessels in vesselless angiosperms.

Symplasmic networks in secondary vascular tissues: parenchyma distribution and activity supporting long-distance transport

Stems that develop secondary vascular tissue (i.e. xylem and phloem derived from the vascular cambium) have unique demands on transport owing to their mass and longevity. Transport of water and assimilates must occur over long distances, while the increasing physical separation of xylem and phloem requires radial transport. Developing secondary tissue is itself a strong sink positioned between xylem and phloem along the entire length of the stem, and the integrity of these transport tissues must be maintained and protected for years if not decades. Parenchyma cells form an interconnected three-dimensional lattice throughout secondary xylem and phloem and perform critical roles in all of these tasks, yet our understanding of their physiology, the nature of their symplasmic connections, and their activity at the symplast-apoplast interface is very limited. This review highlights key historical work as well as current research on the structure and function of parenchyma in secondary vascular tissue in the hopes of spurring renewed interest in this area, which has important implications for whole-plant transport processes and resource partitioning.

Roles of cell walls and intracellular contents in supercooling capability of xylem parenchyma cells of boreal trees

The supercooling capability of xylem parenchyma cells (XPCs) in boreal hardwood species differs depending not only on species, but also season. In this study, the roles of cell walls and intracellular contents in supercooling capability of XPCs were examined in three boreal hardwood species, Japanese beech, katsura tree and mulberry, whose supercooling capability differs largely depending on species and season. XPCs in these species harvested in winter and summer were treated by rapid freezing and thawing (RFT samples) or by RFT with further washing (RFTW samples) to remove intracellular contents from XPCs in order to examine the roles of cell walls in supercooling. RFT samples were also treated with glucose solution (RFTG samples) to examine roles of intracellular contents in supercooling. The supercooling capabilities of these samples were examined by differential thermal analysis after ultrastructural observation of XPCs by a cryo-scanning electron microscope to confirm effects of the above treatments. XPCs in RFTW samples showed a large reduction in supercooling capability to similar temperatures regardless of species or season. On the other hand, XPCs in RFTG samples showed a large increase in supercooling capability to similar temperatures regardless of species or season. These results indicate that although cell walls have an important role in maintenance of supercooling, change in supercooling capability of XPCs is induced by change in intracellular contents, but not by change in cell wall properties.

Velocity and pattern of ice propagation and deep supercooling in woody stems of Castanea sativa, Morus nigra and Quercus robur measured by IDTA

Infrared differential thermal analysis (IDTA) was used to monitor the velocity and pattern of ice propagation and deep supercooling of xylem parenchyma cells (XPCs) during freezing of stems of Castanea sativa L., Morus nigra L. and Quercus robur L. that exhibit a macro- and ring-porous xylem. Measurements were conducted on the surface of cross- and longitudinal stem sections. During high-temperature freezing exotherms (HTEs; -2.8 to -9.4°C), initial freezing was mainly observed in the youngest year ring of the sapwood (94%), but occasionally elsewhere (older year rings: 4%; bark: 2%). Initially, ice propagated rapidly in the largest xylem conduits. This resulted in a distinct freezing pattern of concentric circles in C. sativa and M. nigra. During HTEs, supercooling of XPCs became visible in Q. robur stems, but not in the other species that have narrower pith rays. Intracellular freezing of supercooled XPCs of Q. robur became visible by IDTA during low-temperature freezing exotherms (<-17.4 °C). Infrared differential thermal analysis revealed the progress and the two-dimensional pattern of XPC freezing. XPCs did not freeze at once, but rather small cell groups appeared to freeze at random anywhere in the xylem. By IDTA, ice propagation and deep supercooling in stems can be monitored at meaningful spatial and temporal resolutions.

A comparative ultrastructural study of pit membranes with plasmodesmata associated thickenings in four angiosperm species

Recent micromorphological observations of angiosperm pit membranes have extended the number and range of taxa with pseudo-tori in tracheary elements. This study investigates at ultrastructural level (TEM) the development of pseudo-tori in the unrelated Malus yunnanensis, Ligustrum vulgare, Pittosporum tenuifolium, and Vaccinium myrtillus in order to determine whether these plasmodesmata associated thickenings have a similar developmental pattern across flowering plants. At early ontogenetic stages, the formation of a primary thickening was observed, resulting from swelling of the pit membrane in fibre-tracheids and vessel elements. Since plasmodesmata appear to be frequently, but not always, associated with these primary pit membrane thickenings, it remains unclear which ultrastructural characteristics control the formation of pseudo-tori. At a very late stage during xylem differentiation, a secondary thickening is deposited on the primary pit membrane thickening. Plasmodesmata are always associated with pseudo-tori at these final developmental stages. After autolysis, the secondary thickening becomes electron-dense and persistent, while the primary thickening turns transparent and partially or entirely dissolves. The developmental patterns observed in the species studied are similar and agree with former ontogenetic studies in Rosaceae, suggesting that pseudo-tori might be homologous features across angiosperms.

Gene expression associated with increased supercooling capability in xylem parenchyma cells of larch (Larix kaempferi)

Xylem parenchyma cells (XPCs) in larch adapt to subfreezing temperatures by deep supercooling, while cortical parenchyma cells (CPCs) undergo extracellular freezing. The temperature limits of supercooling in XPCs changed seasonally from -30 degrees C during summer to -60 degrees C during winter as measured by freezing resistance. Artificial deacclimation of larch twigs collected in winter reduced the supercooling capability from -60 degrees C to -30 degrees C. As an approach to clarify the mechanisms underlying the change in supercooling capability of larch XPCs, genes expressed in association with increased supercooling capability were examined. By differential screening and differential display analysis, 30 genes were found to be expressed in association with increased supercooling capability in XPCs. These 30 genes were categorized into several groups according to their functions: signal transduction factors, metabolic enzymes, late embryogenesis abundant proteins, heat shock proteins, protein synthesis and chromatin constructed proteins, defence response proteins, membrane transporters, metal-binding proteins, and functionally unknown proteins. All of these genes were expressed most abundantly during winter, and their expression was reduced or disappeared during summer. The expression of all of the genes was significantly reduced or disappeared with deacclimation of winter twigs. Interestingly, all but one of the genes were expressed more abundantly in the xylem than in the cortex. Eleven of the 30 genes were thought to be novel cold-induced genes. The results suggest that change in the supercooling capability of XPCs is associated with expression of genes, including genes whose functions have not been identified, and also indicate that gene products that have been thought to play a role in dehydration tolerance by extracellular freezing also have a function by deep supercooling.

Plants in a cold climate

Plants are able to survive prolonged exposure to sub-zero temperatures; this ability is enhanced by pre-exposure to low, but above-zero temperatures. This process, known as cold acclimation, is briefly reviewed from the perception of cold, through transduction of the low-temperature signal to functional analysis of cold-induced gene products. The stresses that freezing of apoplastic water imposes on plant cells is considered and what is understood about the mechanisms that plants use to combat those stresses discussed, with particular emphasis on the role of the extracellular matrix.

Cryo-scanning electron microscopic study on freezing behavior of xylem ray parenchyma cells in hardwood species

Differential thermal analysis (DTA) has indicated that xylem ray parenchyma cells (XRPCs) of hardwood species adapt to freezing of apoplastic water either by deep supercooling or by extracellular freezing, depending upon the species. DTA studies indicated that moderately cold hardy hardwood species exhibiting deep supercooling in the XRPCs were limited in latitudinal distribution within the -40 degrees C isotherm, while very hardy hardwood species exhibiting extracellular freezing could distribute in colder areas beyond the -40 degrees C isotherm. Predictions based on the results of DTA, however, indicate that XRPCs exhibiting extracellular freezing may appear not only in very hardy woody species native to cold areas beyond the -40 degrees C isotherm but also in less hardy hardwood species native to tropical and subtropical zones as well as in a small number of moderately hardy hardwood species native to warm temperate zones. Cryo-scanning electron microscopic (cryo-SEM) studies on the freezing behavior of XRPCs have revealed some errors in DTA. These errors are originated mainly due to the overlap between exotherms produced by freezing of water in apoplastic spaces (high temperature exotherms, HTEs) and exotherms produced by freezing of intracellular water of XRPCs by breakdown of deep supercooling (low temperature exotherms, LTEs), as well as to the shortage of LTEs produced by intracellular freezing of XRPCs. In addition, DTA results are significantly affected by cooling rates employed. Further, cryo-SEM observations, which revealed the true freezing behavior of XRPCs, changed the previous knowledge of freezing behavior of XRPCs that had been obtained by freeze-substitution and transmission electron microscopic studies. Cryo-SEM results, in association with results obtained from DTA that were reconfirmed or changed by observation using a cryo-SEM, revealed a clear tendency of the freezing behavior of XRPCs in hardwood species to change with changes in the temperature in the growing conditions, including both latitudinal and seasonal temperature changes. In latitudinal temperature changes, XRPCs in less hardy hardwood species native to tropical and subtropical zones exhibited deep supercooling to -10 degrees C, XRPCs in moderately hardy hardwood species native to temperate zones exhibited a gradual increase in the supercooling ability to -40 degrees C from warm toward cool temperate zones, and XRPCs in very hardy hardwood species native to boreal forests exhibited extracellular freezing via an intermediate form of freezing behavior between deep supercooling and extracellular freezing. In seasonal temperature changes, XRPCs in hardwood species native to temperate zones changed their supercooling ability from a relatively low degree in summer to a high degree in winter. XRPCs in hardwood species native to boreal forests changed their freezing behavior from deep supercooling to -10 degrees C in summer to extracellular freezing in winter. These results indicate that the freezing behavior of XRPCs in hardwood species tends to shift gradually from supercooling of -10 degrees C, to a gradual increase in the deep supercooling ability to -40 degrees C or less, and finally to extracellular freezing as a result of cold acclimation in response to both latitudinal and seasonal temperature changes. It is thought that these temperature-dependent changes in the freezing behavior of XRPCs in hardwood species are mainly controlled by changes in cell wall properties, although no distinct changes were detected by electron microscopic observations in cell wall organization between hardwood species or between seasons. Evidence of temperature-dependent changes in the freezing behavior of XRPCs in hardwood species provided by the results of studies using a cryo-SEM has indicated the need for further investigation to clarify cold acclimation-induced cell wall changes at the sub-electron microscopic level in order to understand the mechanisms of freezing adaptation.

Effect of Macerase, Oxalic Acid, and EGTA on Deep Supercooling and Pit Membrane Structure of Xylem Parenchyma of Peach

The object of this study was to determine if calcium cross-linking of pectin in the pit membrane of xylem parenchyma restricts water movement which results in deep supercooling. Current year shoots of ;Loring' peach (Prunus persica) were infiltrated with oxalic acid or EGTA solutions for 24 or 48 hours and then either prepared for ultrastructural analysis or subjected to differential thermal analysis. The effect of 0.25 to 1.0% pectinase (weight/volume) on deep supercooling was also investigated. The use of 5 to 50 millimolar oxalic acid and pectinase resulted in a significant reduction (flattening) of the low temperature exotherm and a distinct swelling and partial degradation of the pit membrane. EGTA (10 millimolar) for 24 or 48 hours shifted the low temperature exotherm to warmer temperatures and effected the outermost layer of the pit membrane. A hypothesis is presented on pectin-mediated regulation of deep supercooling of xylem parenchyma.

Mediation of deep supercooling of peach and dogwood by enzymatic modifications in cell-wall structure

Treatment of stem sections of peach (Prunus persica (L.) Batsch) and flowering dogwood (Cornus florida L.) with macerase, an enzyme mixture rich in pectinase, for 24-48 h resulted in a complete flattening of the low-temperature exotherm (LTE) as determined by differential thermal analysis (DTA). Ultrastructural analysis of macerase-treated tissue demonstrated a nearly complete digestion of the pit membrane (black cap and primary cell-wall) of nearly 100% of the xylem-parenchyma cells examined after 48 h of exposure to the enzyme. Additionally, the underlying amorphous layer was partially degraded in up to 57% of the cells examined. The macerase treatment had no visible effect on secondary cell-walls of xylem tissue. In contrast, treatment of stem tissue with cellulysin (mostly cellulase) resulted in a shift of the LTE to warmer temperatures as determined by DTA, and a digestion of only the outermost layer of the pit membrane in nearly 100% of the cells examined, with little or no effect on the underlying layers. Treatment of tissue with 25 mM sodiumphosphate buffer also resulted in a shift of the LTE to warmer temperatures but the shift was not as great as in cellulysin-treated tissue. The shift was associated with a partial degradation of the outermost layer of the pit membrane in dogwood (33-45% of the cells examined) but not in peach (3-7% of the cells). Collectively, the data indicate that pectins may be an integral structural element of the pit membrane and that this portion of the cell-wall, along with the underlying amorphous layer, play a major role in forming a barrier to water movement and growth of ice crystals. This barrier allows xylem parenchyma of some species of woody plants to undergo deep supercooling.